Answer from David Svahn MD

Research area:Hypertension, mood and anxiety distorders and medical education.

Education:BS in Quantitative Biology from MIT and MD from Columbia University College of Physicians & Surgeons

There can be three causes of dizziness. One occurs if your blood pressure drops so low that the brain doesn't get enough blood to stay alert. This leads to fainting, which is nature's way of making you lie down so that the brain is at the same level as the heart and can get the blood it needs. Blood pressure can drop for lots of reasons like bleeding from an injury, or from a heart attack (when the heart can't pump blood normally) but the most common causes are much less dangerous such as standing still on a hot day, or getting really nervous or even seeing someone else feel faint! (That shows the power of the mind.)
The second cause is when you suffer "vertigo" which is the same as motion sickness and is most often due to an infection of the inner ear where our balance center is located. That's why sometimes when you have a bad ear infection you feel unsteady and it seems the world is spinning around you even when it isn't.
The third cause, common in older people, is simply unsteadiness due to a slowing down of all the signals from our eyes, our legs and our blood pressure regulation that normally, when we're young, keeps us able to run and dive, and turn upside down (on a swing for example, without getting unsteady for even a second.